Input: wacom - use hidinput_calc_abs_res instead of duplicating its code

This may infer a small difference with the previous implementation
due to the DIV_ROUND_CLOSEST() in the hid implementation.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jason Gerecke <killertofu@gmail.com>
Tested-by: Jason Gerecke <killertofu@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>